If it hadn't been for your help,我们不可能在这门短的时间里就在这实验上取得成功（coul

如果没有你的帮助,我们不可能在这门短的时间里就在这实验上取得成功.
If it hadn't been for your help, we wouldn't have succeeded in the experiment in such a short time.

你翻译的不错,a success 最好改成 succeeded,这样比较地道点.
然后英文里一般都是 if it hadn't been / wouldn't have... 所以把 couldn't 改成 wouldn't.

这样就 perfect 了. 希望对你有帮助—— ♥

建议改为：we couldn't have succeeded in the experiment in such a short time.
题目要求用couldn't have done的句式，done是要用动词的过去分词，而你的句子中用的是名词success，不符合题目的要求，应该用动词succeed 的过去分词。
另外， experiment要用定冠词，在这门短的时间里是...

we couldn't have succeed in the experiment in such a short time